Chlorine dioxide is manufactured on a technical scale, by reacting sodium chlorate in aqueous solution with a suitable reductant. Conventional reductants are sulfur dioxide, sodium chloride, hydrogen chloride and methanol, although other inorganic substances are also mentioned in the literature, such as nitrogen dioxide and sulfur, or organic substances such as ethanol and oxalic acid. Experience and scientific research have shown that the reduction of the chlorate shall be carried out in a strongly acid solution. Sulfuric acid or hydrochloric acid (hydrogen chloride) are the substances most commonly used to effect such acidification.
Subsequent to the chlorate having been consumed, there remains a strongly acid solution of sulfuric acid and sodium, or hydrochloric acid and sodium chloride, so-called spent acid. Sulfur containing spent acid has hitherto been used as a make-up chemical in the pulp mills, wherein the acid has been utilized to cleave or split tall oil soap for the production of tall oil, and the sodium is used to make up alkali losses and sulfur losses in the cooking chemical system of the pulp mills.
A modern variant of the chlorine dioxide process is based on reacting a mixture of sodium chloride and sodium chlorate in a solution which is acidified with sulfuric acid. The reaction takes place under vacuum conditions, 10-57 kPa, at a temperature of 45.degree.-85.degree. C. A mixture of water vapour (steam), chlorine dioxide and chlorine departs during this reaction. The reaction is represented by the formula: EQU NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 =ClO.sub.2 +1/2Cl.sub.2 +Na.sub.2 SO.sub.4 +H.sub.2 O
In view of present-day requirements on the limited use of chlorine and hypochlorite in pulp bleaching processes, it is desirable to limit the amount of molecular chlorine formed. This can be achieved by also using sulfur dioxide as a reductant in accordance with the formula: EQU NaClO.sub.3 +1/2SO.sub.2 =ClO.sub.2 +1/2Na.sub.2 SO.sub.4
Sulfur dioxide can also be used to absorb in an aqueous solution chlorine gas which has not dissolved in the water washing tower used to produce chlorine dioxide water. This results in a mixed acid according to the formula: EQU SO.sub.2 +Cl.sub.2 +2H.sub.2 O=2HCl+H.sub.2 SO.sub.4
This mixed acid car, be returned to the chlorine dioxide reactor. Flows of sodium chlorate, sodium chloride and sulphuric acid are delivered to the reactor. The reaction products chlorine dioxide and chlorine are absorbed in a tower to which pure water is delivered and which produces a liquid which contains chlorine dioxide and chlorine. The residual gas from the absorption tower is passed to a reaction vessel, to which sulfur dioxide and water are supplied. The resultant mixed acid is passed back to the reactor. Crystals of sodium sulfate (Na.sub.2 SO.sub.4) formed in the chlorine dioxide reactor are pumped to a filter and ejected, whereas the mother liquor is returned to the reactor.
According to one alternative embodiment, methanol is used as a reductant, which is supplied to the reactor along with sodium chlorate and sulfuric acid. The chlorine dioxide gas formed is passed to a tower in which the gas is absorbed in water, and the resultant bleaching liquid is passed to the bleaching department. Crystals of sodium sesquisulfate (Na.sub.3 H[SO.sub.4 ].sub.2) formed in the chlorine dioxide reactor are pumped to a filter and ejected, whereas the mother liquor is returned to the reactor. The following reaction formulas are representative of this embodiment: EQU 6NaClO.sub.3 +CH.sub.3 OH+4H.sub.2 SO.sub.4 =6ClO.sub.2 +CO.sub.2 +2Na.sub.3 H(SO.sub.4).sub.2 +5H.sub.2 O
respectively EQU 12NaClO.sub.3 +3CH.sub.3 OH+8H.sub.2 SO.sub.4 =12ClO.sub.2 +3HCOOH+4Na.sub.3 H(SO.sub.4).sub.2 +9H.sub.2 O